This invention relates to an improved method of laser induced photothermal displacement spectroscopy (LIPDS) which is suitable for measuring the absorption spectra of solutions such as those of highly radioactive samples by remote control in a non-contact manner.
According to the invention, the chemical state and concentration of a specific ion can theoretically be measured in a selective manner and, in addition, the measurement is real-time, continuous and automatic. Therefore, the method of the invention has a particular advantage in that uranium, transuranic elements, nuclear fission products, corrosion products, etc. in solutions that occur in the reprocessing of spent fuels from nuclear power generators, as well as in highly radioactive liquid wastes can be analyzed either qualitatively or quantitatively by an in-line process (remote controlled and in a non-contact manner) in accordance with the specific chemical state of ion. Stated more specifically, not only the oxidation state of radioactive ions but also various complexes thereof can be analyzed by selective measurements of the chemical states and concentrations of ions.
Absorptiometric analysis is a conventionally employed method for measuring the absorption spectrum of solutions. According to this method, the absorption of light that has passed through a sample is measured and the absorption wavelength of the sample provides for its qualitative analysis whereas the absorbance its quantitative analysis. The method has various salient features such as the ability to perform selective measurement and analysis of ion's chemical state and concentration, a capability for nondestructive analysis, rapidity in measurement and ease of operations in measurement.
While the chemical state and concentration of a particular ion can be selectively measured by several methods of analysis, absorptiometric analysis is the only method that can be applied in-line. Therefore, with a view to analyzing uranium and plutonium for achieving the accountability of nuclear fuels and the process control of facilities in the reprocessing of spent fuels from nuclear power generators, as well as for the purpose of monitoring the state of neptunium and other transuranic elements and for controlling their behavior, the development of a technology for in-line analysis by the absorptiometric approach is underway and there is an actual case of application of this method in the analysis of uranium.
Photothermal spectroscopy is known to be capable of measuring the chemical states and concentrations of ions in solution with higher sensitivity than absorptiometric analysis and examples of this technique include photoacoustic spectroscopy, thermal lens spectroscopy and photothermal deflection spectroscopy. In absorptiometric analysis, the intensity of light transmission through a sample solution is taken as relative to the light transmittance through a reference solution, so the sensitivity of this method is not dependent on the intensity of the light source. In contrast, photothermal conversion analysis measures the absorbed light energy, so the signal intensity obtained in this method is dependent on the intensity of the light source. Therefore, in photothermal conversion analysis, a high-intensity light source such as laser light is used as an exciting light source to thereby provide a higher sensitivity in measurement than absorptiometric analysis.
Photothermal conversion analysis is performed on two different principles; in one case, the light energy absorbed by a sample solution is converted to heat by non-radiation transition and the resulting elastic wave is detected with a piezoelectric device (as in photoacoustic spectroscopy); in the other case, a refractive index profile due to the thermal distribution produced by photothermal conversion is detected by optical means (as in thermal lens spectroscopy and photothermal deflection spectroscopy). However, either approach still remains a subject of basic research and no practical case has yet materialized in the actual process.
If absorptiometric analysis is to be applied to in-line analysis, fiber optics has to be used not only for introducing excitation light but also to detect transmitted light and this makes it necessary to consider the radiation resistance of fiber optics in a high radiation field. In photoacoustic spectroscopy which involves detection with a piezoelectric device, the introduction of excitation can be remotely controlled by using a laser but, on the other hand, the sample or sample container must be brought into direct contact with the piezoelectric device. Thermal lens spectroscopy and photothermal deflection spectroscopy have a possibility for realizing non-contact measurements by remote control but this approach is still at the stage of basic research.